Sodium hydroxide and chlorine which are important as industrial raw materials have been mainly produced by a brine electrolysis process.
The electrolysis process has made the shift to an ion-exchange membrane process using an ion-exchange membrane as a diaphragm and using an activated cathode having a low overvoltage, via a mercury process using a mercury cathode and a diaphragm process using an asbestos diaphragm and a soft-iron cathode. During this interval, the electric power consumption rate required for the production of 1 ton of caustic soda has decreased to 2,000 kWh.
The activated cathode is obtained, for example, by using the following methods or materials.
That is, the method includes a method of obtaining an activated electrode by dispersing a ruthenium oxide powder in a Ni plating bath and carrying out composite plating, a method of obtaining an activated electrode by NiO plating containing a second component such as S or Sn, and a method of obtaining an activated electrode by NiO plasma splaying or Pt—Ru immersion plating, and the material includes Raney nickel, a Ni—Mo alloy and a hydrogen storing alloy for imparting resistance to reverse current (H. Wendt, Electrochemical Hydrogen Technologies, pp. 15-62, 990, U.S. Pat. No. 4,801,368, J. Electrochem. Soc., 137, 1419 (1993), Modern Chlor-Alkali Technology, vol. 3, 1986).
In Japanese Patent Nos. 1,911,015 and 1,911,016, it is reported that a mixed catalyst of cerium and a noble metal has resistance to iron contamination. Recently, in the ion-exchange membrane electrolytic process, electrolysis cells which can increase current density for an increase in production capacity and a decrease in investment cost have been devised, and the load of a high current has become possible by development of low-resistant membranes.
DSA as an anode has a running record up to 200 to 300 A/dm2 in the mercury process. Although there is no record yet for the life and performance of a cathode in the ion-exchange membrane process, requests for improvement as described below have been made.
That is, it has been required that the overvoltage is low, that the cathode does not damage a membrane when it comes into contact with the membrane, and that the contamination due to metal ions and the like from the cathode is small.
When these improvements are not cried out, it becomes difficult to use the cathode which has hitherto been used (having large concaves and convexes on a surface thereof and having a catalytic layer with low mechanical strength. In order to realize a new process, the development of an activated cathode having high performance and sufficient stability even under the above-mentioned electrolysis conditions is also indispensable.
In the brine electrolysis process using an activated cathode, which has been most generally conducted at present, the cathode is arranged in contact with a cation-exchange membrane on the cathode side thereof or with a gap of 3 mm or less. When water reacts in a catalytic layer to form sodium hydroxide, an anode reaction and a cathode reaction are each as shown below, and the theoretical decomposition voltage thereof is 2.19 V.2Cl−═Cl2+2e(1.36 V)2H2O+2e=2OH−+H2(−0.83 V)
However, when the conventional activated cathode is used for running at high current density, it involves several significant problems as follows.
(1) Associated with deterioration of the electrode, a substrate (nickel, iron and carbon components) is partially dissolved and comes away to move to a cathode solution, a membrane or an anode chamber, which causes a decrease in product quality and deterioration of electrolytic performance.
(2) The overvoltage increases with an increase in current density, resulting in a decrease in energy efficiency.
(3) The bubble distribution in an electrolyzer increases with an increase in current density to cause the occurrence of distribution in the concentration of sodium hydroxide formed, so that the solution resistance loss of the cathode solution increases.
(4) When the operating conditions become severe, the elution of impurities (such as sulfur and iron) from cell-constituting materials increases to contaminate the electrode.
Further, the arrangement of the cathode in close contact with the ion-exchange membrane (zero gap) is desirable because this should make the voltage lower. However, there is a concern that the membrane may be mechanically broken by the cathode having a roughened surface shape. Hence, there has been a problem in the use of the conventional cathode at high current density under the zero gap conditions.
A cathode using a noble metal as the catalyst has also hitherto been proposed, and is promising in its performance. However, it has a price problem, and the amount thereof used should be necessarily reduced. In this case, the catalytic layer becomes thin, so that the substrate is easily dissolved and comes away. Accordingly, improvement thereof has also been desired.
JP-A-2000-277966 discloses a method for producing a cathode comprising ruthenium and cerium by using a coating solution to which oxalic acid is added. JP-A-2006-299395 discloses a method for producing a cathode comprising three components of ruthenium, cerium and niobium. JP-A-2006-118022 and JP-A-2006-118023 disclose a method for producing a cathode comprising an alloy of platinum and an iron group element, copper or silver.    Patent Document 1: JP-A-2000-239882